A technique is known, wherein a particulate filter (hereinafter simply referred to as “filter”), which traps the particulate matter (hereinafter referred to as “PM”) contained in an exhaust gas, is provided in an exhaust gas passage of an internal combustion engine.
A technique is disclosed in Patent Document 1, which is usable when a PM trapper (corresponding to the filter) is provided in an exhaust gas passage of an internal combustion engine, wherein any breakdown or failure of the PM trapper is detected on the basis of an amount of change of the differential pressure between the upstream and the downstream of the PM trapper brought about in accordance with the change of the operation state of the internal combustion engine.
A technique is disclosed in Patent Document 2, wherein any filter breakdown or failure is checked or diagnosed on the basis of a detected value of a particulate amount detection sensor provided in an exhaust gas passage at a portion disposed on the downstream side from a filter. In the technique described in Patent Document 2, the breakdown check for the filter is prohibited if a condition, under which the PM trapping efficiency of the filter is decreased to be lower than a predetermined value, is established.
Further, the following fact is described in Patent Document 2. That is, PM, which has a particle size (grain diameter) smaller than a mesh size of the filter, passes through the filter during a certain period of time after the regeneration or renewal. Therefore, the PM trapping efficiency of the filter is lowered. When a certain amount of PM is accumulated or deposited, and the meshes of the filter are progressively filled therewith, then the PM trapping efficiency is recovered.